Lithium silicate materials are known for a long time and used for various applications, in particular in the dental area for producing dental restorations.
US 2010/0093706 (Castillo) and US 2009/0256274 (Castillo) relate to lithium silicate glass ceramic materials for the manufacture of blocks for dental appliances using a CAD/CAM process. The lithium silicate materials include from 1 to 10 wt.-% germanium dioxide. The materials are said to have improved castability and higher density.
US 2009/0042717 (Apel et al.) describes lithium silicate materials which are said to be easily millable and having a high strength. The lithium silicate materials contain Me(II)O being selected from CaO, BaO and SrO and less than 0.1 wt.-% ZnO.
U.S. Pat. No. 6,420,288 B2 (Schweiger et al.) deals with a process for the preparation of shaped translucent lithium disilicate glass ceramic products, which comprises producing a melt of a starting glass containing SiO2 (57.0 to 80.0 wt.-%), Al2O3 (0 to 5.0 wt.-%), La2O3 (0.1 to 6.0 wt.-%), MgO (0 to 5.0 wt.-%), ZnO (0 to 8.0 wt.-%) and Li2O (11.0 to 19.0 wt.-%). It is mentioned that the additional incorporation of ZrO2 let to an increase in translucency.
US 2005/0054509 (Hoen et al.) relates to a translucent and radio-opaque glass ceramic which may comprise Li2O from 0 to 3 wt.-%, Na2O from 0 to 9.0 wt.-%, K2O from 3.0 to 14.0 wt.-%, Rb2O from 0 to 12.5 wt.-% and Cs2O from 0 to 18.0 wt.-%. It is stated that the main crystalline phase is apatite, in particular a Ca-apatite where Ca is totally or partially replaced by Sr and/or Mg.
U.S. Pat. No. 6,155,830 (Brodkin et al.) describes a dental restoration comprising a porcelain composition comprising a glassy matrix and leucite crystallites embedded therein. The coefficient of thermal expansion (CTE) is said to be in a range from 12 to 15*10−6/° C. The content of Li2O is in a range of 0 to 3 wt.-%.
EP 2 377 830 A1 describes a lithium silicate glass ceramic comprising at least 8.5 wt.-% transition metal oxides selected from oxides of yttrium and oxides of the transition metals having the numbers 41 to 79.
U.S. Pat. No. 6,133,174 (Brodkin et al.) deals with a method of producing a machinable feldspathic porcelain composition comprising leucite. The content of Li2O of the composition is said to be within a range from 0 to 3 wt.-%. The 3-point bending strength measured is within a range from 127 to 136 MPa.
WO 99/18910 (Jeneric Pentron) relates to a dental porcelain composition comprising a continuous glassy phase and a discontinuous, substantially crystalline phase comprising cubic leucit. The porcelain exhibits a coefficient of thermal expansion in the range of 11 to 17*10−6/° C. The Li2O content of the raw material is said to be from 1.5 to 4 mol.-%.
However, there is still a need for lithium silicate glass ceramic materials which facilitate the production of multi-layered milling blocks comprising a translucent layer of material.